The global hydrogen market has been growing very rapidly, with recent growth rates exceeding 10% per year. Currently, about 95% of the hydrogen used today comes from the reforming of natural gas. In the near future, it is expected that the demand of hydrogen will increase due to the introduction of hydrogen fuel cells.
There are three major processes for hydrogen production: steam reforming (SR) (Equation 1), partial oxidation (POX) (Equation 2) and autothermal reforming (ATR) (Equation 3), as illustrated below.Steam Reforming CH4+H2O→CO+3H2  (1)Partial Oxidation CH4+½O2→CO+2H2  (2)Autothermal Reforming 2CH4+H2O+½O2→2CO+5H2  (3)
Generally, the reformate gases comprise H2, CO and CO2, H2O and a small amount of unconverted fuel. To maximize hydrogen production and avoid poisoning down stream catalyst, carbon monoxide in the reformate gas should be converted to carbon dioxide through the water gas shift (WGS) reaction, as shown below.Water Gas Shift Reaction CO+H2OCO2+H2  (4)
One of the hurdles in the application of fuel processing to generate hydrogen for fuel cells is the lack of an efficient WGS catalyst. The state-of-the-art WGS catalysts are mostly based on Cu—ZnO and Fe3O4—Cr2O3. These catalysts, however, possess slow kinetics, and are very sensitive to temperature excursions and to air exposure. Careful activation is often required for these catalysts. As a result, these catalysts have been deemed unsuitable for the synthesis of hydrogen for fuel cell applications. See, e.g., Song, Chunshan, “Fuel processing for Low-Temperature and High-Temperature Fuel Cells: Challenges, and Opportunities for Sustainable Development in the 21st Century,” 77(1-2) Catalysis Today at 17-49 (2002). Also, these WGS catalysts are highly sensitive to sulfur, which poses a particular challenge for hydrogen production from coal gasification and integrated gasification combined cycle (IGCC). Thus, there is a great need for active, nonpyrophoric, sulfur resistant WGS catalysts.
Precious metals such as platinum, palladium, rhodium and gold supported on oxides have been reported as being active and stable as WGS catalysts over a wide temperature range. See, e.g., S. L. Swartz, M. M. Seabaugh, C. T. Holt and W. J. Dawson, “Fuel Processing Catalysts Based on Nanoscale Ceria,” 30 Fuel Cell Bulletin at 7-10 (2001). Additionally, such WGS catalysts overcome the disadvantages of iron and copper based materials, mentioned above.
There is growing interest in implementing zirconium oxide-based compositions as WGS catalysts because of the unique amphoteric properties associated with the surface hydroxyl groups of zirconium oxide and because of its high thermal stability. Also, zirconia-supported metal catalysts are useful in many chemical processes, such as, for example, in methanol synthesis (particularly zirconia-supported copper) and Fischer-Tropsch synthesis (particularly zirconia-supported nickel) processes.
Zirconia formed from conventional preparation methods normally contains a thermally stable monoclinic crystal structure (m—ZrO2), which is known to transition at about 1200° C. to tetragonal phase (t—ZrO2) and at about 2280° C. to cubic phase (c—ZrO2) crystal structures. The physical and chemical properties of zirconia are closely related to the crystal phases and determine its application in various industries. Of the three phases, tetragonal zirconia has great potential for catalytic applications.
Conventional processes for synthesizing tetragonal zirconia, however, have been unable to form high surface area tetragonal zirconia because of sintering encountered during the heat treatment of monoclinic phase zirconia to form tetragonal phase zirconia.
Thus, the need exists for high surface area tetragonal zirconia compositions and for processes to synthesize such high surface area tetragonal zirconia compositions.